To operate an engine efficiently, it is necessary to warm up the engine to an appropriate temperature after starting the engine. Warming up the engine has been carried out by stopping circulation of a refrigerant which cools the engine and raising the temperature of the engine. In another method, the engine is warmed up quicker by heat exchange between an exhaust gas of the engine and a refrigerant such that the refrigerant is heated using exhaust heat of the engine (e.g., see JP 4826502 B1).
Alternatively, a method has been proposed (e.g., JP 2011-99400 A), in which a valve for adjusting the flow rate of a refrigerant flowing through the engine is provided. Upon cold start of the engine, the valve is first closed to inhibit flowing of the refrigerant in the engine in order to warm up the engine, and when the temperature of the engine is raised to reach a certain level, the valve is opened to allow the refrigerant to flow through the engine while the warming up of the engine is continued. When the warming up of the engine is finished, a normal operation of causing the refrigerant to flow through a radiator to prevent overheating of the engine is carried out. It has also been proposed to use an electromagnetic valve as the valve mentioned above. A voltage is applied to such an electromagnetic valve to decrease a degree of opening of the valve, while the voltage is shut off to increase the degree of opening of the valve (e.g., see JP 2014-1654 A).
Meanwhile, a technique to intermittently stop the engine has been used in many cases as a technique to maximize fuel efficiency or minimize electric power consumption. When the engine is stopped intermittently, it is not necessary to cause the refrigerant to flow in the engine. Accordingly, it has been proposed to stop an electric refrigerant pump together with the engine to decrease the power consumption (e.g., see JP 2010-180713 A).